Method and apparatus for removing tank sealants



July 14, 1959 A. F. SLOV ER ETAL 2,894,861

I METHOD AND APPARATUS FORREMOVING TANK SEALANTS Filed Sept. 9, 1954 v '2 Shee ts-Sheet 1 7 sum TANK REFRIGERAIIDI um'r STORAGE TANK INVENTORS flncuv Java-1e 65020: L. A/neeouz July 14, 19591.

A. F, SLOVER ETAL- METHOD AND APPARATUS FOR. REMOVING TANK SEALANTS Filedsegit. 9, 1954 2 Sheets-Sheet 2 w 6 mnw M mm T 3 x. 3 v mu w izfi 2:9. Q v.52 U338 Maw s :2 v. mm 3 52,25 5: u M 3 1 3 mflfiv.

:dangerous fumes when Working in the tanks.

METHOD AND APPARATUS FOR REMOVING TANK SEALANTS Archy F. Slover, Arcadia, and George L. Harbour, Lakewood, Califi, assignors, by mesne assignments, to Pennsalt Chemicals Corporation, Philadelphia, Pa., a corporation of Pennsylvania Application September 9, 1954, Serial No. 455,049

5 Claims. (Cl. 134-22) sealants are of special composition to resist the effect of thefhighly volatile and corrosive fuels, and are applied as a relatively thick coating. The effective life of these materials varies, but is considerably shorter than that of the aircraft itself. Therefore, at intervals of some several thousand hours of use it is necessary to remove the old sealant and recoat it with fresh material. This process of removing the old sealant is commonly known as descaling, and various methods of removal have been suggested.

Scraping the sealant from the tank surfaces with or without the use of stripper solutions requires an extreme amount of manpower and the workmen are subject to This method is quite unsatisfactory when any considerable area of sealant must 'be removed, and is confined largely to patching jobs. Removal of the sealant by filling the tanks with stripper or desealant, allowing the stripper solution to stand in the tanks for a considerable period of time, and then draining the stripper and the loosened sealanthas also been tried. In this system the soaking time must be quite long, and the operation must be repeated several times to obtain a good job. It is therefore practical only when the aircraft can be kept out of service for a long time. Finally, various complex spraying systems. have been proposed. These require considerable manifolding, and constant manipulation by the operators so that the spray is properly directed and controlled. The cost of operating these systems is therefore quite high and the results not always uniformly good.

The present invention contemplates a recirculating system wherein the stripper solution is constantly circulating through the tanks under a small pressure. The piping network and the tanks are kept full so that the solvent acts in all parts of the tank at all times to thus reduce to a minimum the time required to loosen the sealant. The path of circulation is from one side to the other of the tanks so that there are no pockets or corner traps wherein the stripper is not in active contact with the sealant. Provision is also made for reversing the direction of flow so that backwashing can be done. While such a system is in itself a considerable advantage over previous systems, its major advantage ist-hat it permits accurate temperature control of the process.

We have found, that the action of the stripper or solvent is most effective within a fairly small temperature range which is exceeded by the normally encountered ambient temperature ranges in which the aircraft may be situated. on the one hand, if the skin temperature of the United States Patent 0 aircraft exceeds 120 F. the stripper or solvent will evap- .orate rapidly, and the inhibitors in the shipping solution are much less effective in protecting'the metal surfaces of the tanks. Since the most active ingredients in the stripper evaporate first, continued exposure to extreme temperatures will result in a much less efficient material. On the other hand, if the skin temperature of the aircraft is below F. the action of the stripper is greatly retarded and the time required to do the job increases so much as to make the operation very costly.

In the circulating system which embodies the present invention, a reversible heat exchanger is provided for maintaining the temperature of the stripper well within optimum operating temperature ranges. Since all of the stripping solution is constantly circulating at. a relatively rapid rate, the working temperature of the stripper in the tanks is quite close to that established by the heat exchanger and is well within the optimum range for the material. It should be'particularly noted that a reversible heat exchanger is provided, that is, one which is capable of supplying external heat to cold stripping solution, or extracting heat from hot solution. Cooperating heating and refrigeration units which may be selectively connected to a common heat exchange unit provide for this function.

The manpower required for operating this system is very low since once the process has started, the only control necessary is that over the heat exchanger and the directional valves in the piping network, which require only periodic attention. At the same time, the stripper is kept in the most efiicient condition to complete the operation in a minimum amount of time.

With the foregoing in mind it is a'major object of the invention to provide a method and apparatus for removing tank sealants which is a temperature controlled circulating system.

An equally important object of the invention is to provide circulating apparatus including a reversible heat exchanger unit.

It is also an object of the invention to provide a circulating system adapted to continuously circulate stripping solution through the wing tanksin a flow path which keeps the tanks full ofactive stripping solution and avoids air pockets or dead spaces.

- .A further object of the invention is to provide a circulation system having by-pass means and valve means permitting isolation of various portions of the system for different operations.

Another object of the invention is to provide a system of the character described which is simple and trouble free and may be operated with minimum attention. These and other objects and advantages of the invention will become apparent from the following detailed description of a preferred form thereof and from an inspection of the accompanying drawings in which:

Fig. l is a complete diagrammatic view of the system sholwn in normal operating connection with aircraft wing tan s;

Fig. 2 is a detail of a flow control valve for the system;

and

Fig. 3 is a view similar to Fig. 1 showing the system connected for reverse flow through the tanks.

Referring now to the drawings, a typical aircraft is shown as having integral wing tanks or cells 10 and 11 which are formed by the wing skin surface 12 and internal ribs or bafller 13. Each of the tanks 10 and 11 is normally coated internally with a sealant that prevents leakage through the structural joints. Each tank is also normally provided with an upper filler opening 14 and a lower drain opening 15.

In preparing the aircraft for desealing, vent piping 18 is run into the tanks 10 and 11 with branches extending upwardly into the top corners of the tanks. A shut off valve 19 may be positioned at the bottom of the vent piping 18 as shown. In a preferred form of the system, surge tanks 20 are mounted on the top of the tanks at the filler openings 14, and each surge tank is provided with a sight glass Both surge tanks 20 are connected by branoh piping 22 to a common return line 23, and both lower drain openings 15 are connected by piping 24 to a comlmon intake line 25. Branch pipes 24 are each provided with a flow regulating valve 24a to control the flow therethrough, and to permit any one of the fuel tanks to be cut out of the system. Main intake line '25 also has a control and shut off valve 26 therein ahead of the branches 24. The designation of lines 23 and 25 as a return and intake lines, respectively, indicates the normal flow path of stripper solution, which is upwardly through both tanks and 11. As will later be described, in backfiushing the tanks this flow path is reversed.

By the piping arrangement just described, both tanks 10 and 11 are filled with stripper from the bottom with the vent piping 18 open through valve 19. As tanks 10 and 11 fill, the air entrapped in the top of the tanks is able to escape through the vent piping 18 until all of the air is gone and the vent piping commences to overflow with stripper. Valve 19 is then closed, thus making sure that all of the interior sealant coated surfaces of the tank are in contact with the stripping solution- During the filling of tanks 10 and 11 and the circulation period, sight glasses 21 are checked to regulate the flow through the tanks by means of valves 24a so that both tanks are kept full at all times. Modifications of this portion of the piping system can, of course, be made to fit different structural conditions in different aircraft, and as the number and location of tanks varies.

In order to supply stripping solution to the fuel tanks, the system includes a storage tank 27 of relatively large capacity. Tank 27 has an upper outlet pipe 28 connected to the suction side of a pump 29 which. serves to pump the solution to the fuel tanks 10 and 11 through a piping network as will be described. In accordance with the ambient temperature conditions, the temperature of the stripping solution coming from storage tank 27 may or may not be within the optimum range for most active work on the sealant coating. Therefore, a heat exchanger 30 is directly coupled to the output side of pump 29 to extract or supply heat to the stripping solution. Until the solution temperature has been brought within the optimum working range it should not be put into the fuel tanks, and we therefore provide by-pass means for returning the solution pouring out of the heat exchanger 30 directly back to storage tank 27. As shown, an output pipe 31 from exchanger 30 is connected to a valve 32 which has two outlet parts. One part is connected directly to the system input line 25, and the other is connected to a by-pass return line 34 which returns to storage tank 27 through a shut off valve 35. By reference to Figs. 1 and 2, the two positions of valve 32 are seen. In Fig. 1, valve 32 is positioned with an inner rotatable spool 36 so aligned as to connect heat exchanger pipe 30 with system intake 25 through an upper connecting port 37. In Fig. 2, spool 36 is rotated, so that a lower port 38 is positioned to align heat exchanger pipe 39 with return line 34, and port 37 is ineffective. Any typeof two way valve means may, of course, be used for valve 32, that shows being merely typical.

Heat exchanger 30 may be of tube and drum type having provision for pouring the stripper solution therethrough independent of a circulating heating or cooling medium. As shown, circulating piping 40 leads to exchanger 30 through shut off valves 41 from an upper refrigerating unit 42. Circulating piping 43 similarly leads to exchanger 30 from a heating unit 44 through shut ofi? valves 45. By this arrangement, either a hot or cold circulating medium or fluid may be sent to exchanger 30 for imparting or extracting heat from the stripper solution. Since only one unit is in use at a time, the other unit is shut off from the heat exchanger 30 through the appropriate valves 41 or 45. These valves 41 and 45 may also be used to regulate the amount of fluid circulation through the heat exchanger 30 so that the output temperature of the stripping solution can be controlled within close limits.

Once the stripping solution contained within the storage tank 27 has been brought to the proper temperature, by-pass line 34 is closed, and the solution is pumped into fuel tanks 10 and 11 through the intake line 25. These tanks are filled in the manner previously described, and as the solution continues to flow into the tanks 10 and 11, it is taken out of the tanks through the return line 23. Circulation through the tanks is maintained continuously for a sufficient length of time to loosen the sealant from the tank walls. As the sealant is loosened it pours out through return line 23 with the stripping solution.

Solution returning through line 23 enters a sump tank 50 where most of the sealant settles out of the stripper solution. A pump 51 takes the solution out of sump tank 59 and returns it to storage tank 27 through an outlet pipe 52 which is connected to a storage return line 53. Connected into line 53 ahead of storage tank 27 are a series of strainers 54 which remove any residual sealant or other foreign material from the stripping solution before it is returned to the storage tank. Thus, the stripper solution again circulated back into the fuel tanks from storage tank 27 is clean and is therefore most efficient in further loosening the tank sealant. As can be appreciated, the system just described is a closed system; providing for continuous circulation of stripper solution through the fuel tanks for as long a time as is necessary to complete the removal of the sealant.

In order to prevent excessive pressure from being applied to the fuel tanks 1%) and 11, a pressure regulating standpipe 58 is connected between input line 25 and sump tank 50. As shown, a line 59 branches. off from line 25 at a T-connection 60. Line 59 is a drain line leading to tank 50 through a normally closed valve 61, and is used in backwashing and draining the fuel tanks as will be described. In line 59 ahead of valve 61 is a T-connection 62 for joining standpipe 58. The latter is elevated in a closed loop and then return to tank 50. Normally, there is no how through standpipe 58 because of the back pressure presented. However, should the back pressure in the fuel tanks 10 and 11 exceed the back pressure of standpipe 53, the latter provides a bypass flow path to prevent blowup or injury to the fuel tanks.

During the circulation of the stripper solution the temperature thereof is controlled by heat exchanger 30 for most effective action. As the ambient temperature changes, adjustments are periodically made in the circulation of the heating or cooling medium so as to hold the temperature of the solution within the optimum range. It should also be noted that the flow path of stripper solution through the fuel tanks is from bottom to top so that there are no pockets or dead spaces within the tanks and fresh solution is being continuously supplied for action on the sealant.

After the circulation has continued for a sufficient time to loosen the tank sealant it may be desirable to backflush the fuel tanks 10 and 11 by reverse circulation, or merely to drain the stripper solution into the sump tank. The system herein disclosed permits either of these operations. It should be noted, that in either backflushing or draining the fuel tanks, the stripper solution will be collecting the residual sealant on the tank walls and it is not desirable to put this solution back into the storage tank.

In backflushing, the sump pump 51 is used as the circulation pump, and the storage tank 27 and heat e'xchanger 30 are isolated from the system. This is done by closing valve 26 in line 25 and a similar valve 63 in return line 53. Line 52 extends past return line 53 to join line 23 and has a normally closed valve 64 therein. In backflushing, valve 64 is opened and pump 51 picks up fluid from sump tank 50 forcing it out through line 52 which joins to line 23 at a three way connection 65. A shut off valve 66 in line 23 near the entrance to tank 50 prevents by-passing the fluid back into the tank, and is closed at this time. Also joined to connection 65 is a by-pass standpipe 67 which serves the same function as the main by-pass standpipe 58.

Unless the back pressure on the tank exceeds that of standpipe 67 no solution flows through this line, but is forced up towards the fuel tanks and -11 through line 23.

The flow diagram for backflushing is shown in Fig. 3, and as can be seen the stripper solution is pumped into the top of tanks 10 and 11 and out the bottom through lines 24 into line 25. The previously mentioned valve 61 in drain line 59 is opened to thus provide a direct path from line 25 back to sump tank 50. Backflushing circulation is thus continued, until all of the residual sealant is removed from the walls of tanks 10 and 11.

Should it be desirable merely to drain fuel tanks 10 and 11 without backflushing the sump pump 51 can be shut down to allow the solution to drain gravitationally from the fuel tanks into sump tank 50 through drain line 59 and valve 61.

It can therefore be understood that the preferred apparatus for carrying out the invention is quite flexible in operation to permit the descaling to be accomplished with a minimum of time and expense. Modifications of design and construction can be made without departing from the principles of the invention, and We do not wish to be restricted to the foregoing detailed description except as defined in the appended claims.

We claim:

1. The method of removing sealant from aircraft fuel tanks which consists of venting said tanks from the top; filling said tanks with solvent from the bottom until the vents overflow and said tanks are completely full; closing said vents; circulating said solvent through said tanks from bottom to top under sufficient pressure to keep said tanks completely full; externally passing said solvent through a reversible heat exchanger to maintain said solvent at an optimum working temperature; and reversing the direction of flow of said solvent; and circulating said solvent through a part of the circulating system isolated from said heat exchanger.

2. Apparatus for removing sealant from aircraft fuel tanks which includes: a closed piping system connected to the top and bottom of said tanks for circulating solvent therethrough, said system including a storage tank and a sump tank; pump means in said system for forcing said solvents through said fuel tanks from bottom to top under sufllcient pressure to keep said fuel tanks completely full; and a reversible heat exchanger in said system for maintaining said solvent at an optimum working temperature; pump means in said sump tank for pumping said solvent out of said sump tank to said storage tank; and bypass valve means in said system for reversing the flow of solvent through said fuel tanks and by-passing said heat exchanger and said storage tank, said sump pump means being used to force the flow of solvent in the reverse direction.

3. Apparatus for removing sealant from aircraft fuel tanks which includes: a closed piping system connected to the top and bottom of said tanks for circulating solvent therethrough, said system including a storage tank and a sump tank; strainer means in said system between said sump tank and said storage tank; independent venting means leading downwardly and outwardly from the top of said fuel tanks; pump means in said system for forcing said solvents through said fuel tanks from bottom to top under sufiicient pressure to keep said fuel tanks completely full; a heat exchanger in said system for maintaining said solvent at an optimum working temperature; means connectible with said heat exchanger for circulating a heating medium therethrough; means connectible with said heat exchanger for circulating a cooling medium therethrough; valve means in said system for directing the circulation of said solvent between said storage tank and said heat exchanger for raising and lowering the temperature of said solvent to the optimum working temperature prior to filling said fuel tanks; pump means in said sump tank for pumping said solvent out of said sump tank to said storage tank; and by-pass valve means in said system for reversing the flow of solvent through said tanks and by-passing said heat exchanger and said storage tank, said sump pump means being used to force the flow of solvent in the reverse direction.

4. Apparatus for removing sealant from aircraft fuel tanks which includes: a piping system connected to said fuel tanks for circulating solvent therethrough from the bottom, said system including a storage tank and a sump tank; pump means in said system for forcing said solvent through all of said tanks; a reversible heat exchanger in said system for maintaining said solvent at an optimum working temperature; valve means in said system for circulating said solvent between said storage tank and said heat exchanger for raising or lowering the temperature of said solvent to the optimum working temperature prior to entrance into said fuel tanks; and valve means insaid system for directing the circulation of said solvent through said storage tank, said fuel tanks, said sump tank and said heat exchanger for maintaining said solvent at said optimum working temperature during the removal of said sealant.

5. The method of removing sealant from aircraft fuel tanks which consists of: venting said tanks from the top; circulating said solvent through a reversible heat exchanger prior to entrance into said fuel tanks for raising or lowering the temperature of said solvent to the optimum working temperature; passing said solvent through said fuel tanks from the bottom and thereafter through said heat exchanger for maintaining said solvent at the optimum working temperature during removal of said solvent; and recirculating said solvent back through said fuel tanks.

References Cited in the file of this patent UNITED STATES PATENTS 2,222,516 Powell Nov. 19, 1940 2,344,555 McGrath Mar. 21, 1944 2,653,116 Whitcomb Sept. 22, 1953 2,690,327 Sardeson Sept. 28, 1954 

1. THE METHOD OF REMOVING SEALANT FROM AIRCRAFT FUEL TANKS WHICH CONSISTS OF VENTING SAID TANKS FROM THE TOP; FILLING SAID TANKS WITH SOLVENT FROM THE BOTTOM UNTIL THE VENTS OVERFLOW AND SAID TANKS ARE COMPLETELY FULL; CLOSING SAID VENTS; CIRCULATING SAID SOLVENT THROUGH SAID TANKS FROM BOTTOM TO TOP UNDER SUFFICIENT PRESSURE TO KEEP SAID TANKS COMPLETELY FULL; EXTERNALLY PASSING SAID SOLVENT THROUGH A REVERSIBLE HEAT EXCHANGE TO MAINTAIN SAID SOLVENT AT AN OPTIMUM WORKING TEMPERATURE; AND REVERSING THE DIRECTION OF FLOW OF SAID SOLVENT; AND CIRCULATING SAID SOLVENT THROUGH A PART OF THE CIRCULATING SYSTEM ISOLATED FROM SAID HEAT EXCHANGER.
 2. APPARATUS FOR REMOVING SEALANT FROM AIRCRAFT FUEL TANKS WHICH INCLUDES: A CLOSED PIPING SYSTEM CONNECTED TO THE TOP AND BOTTOM OF SAID TANKS FOR CIRCULATING SOLVENT THERETHROUGH, SAID SYSTEM INCLUDING A STORAGE TANK AND A SUMP TANK; PUMP MEANS IN SAID SYSTEM FOR FORCING SAID SOLVENTS THROUGH SAID FUEL TANKS FROM BOTTOM TO TOP UNDER SUFFICIENT PRESSURE TO KEEP SAID FUEL TANKS COMPLETELY FULL; AND A REVERSIBLE HEAT EXCHANGER IN SAID SYSTEM FOR MAINTAINING SAID SOLVENT AT AN OPTIMUM WORKING TEMPERATURE; PUMP MEANS IN SAID SUMP TANK FOR PUMPING SAID SOLVENT OUT OF SAID SUMP TANK TO SAID STORAGE TANK; AND BYPASS VALVE MEANS IN SAID SYSTEM FOR REVERSING THE FLOW OF SOLVENT THROUGH SAID FUEL TANKS AND BY-PASSING SAID HEAT EXCHANGER AND SAID STORAGE TANK, SAID SUMP PUMP MEANS BEING USED TO FORCE THE FLOW OF SOLVENT IN THE REVERSE DIRECTION. 